Method for operating a regenerative gas heater, and a gas heater for use in this method

ABSTRACT

In a hot blast stove for a blast furnace a method and means are provided to prevent, during the &#39;&#39;&#39;&#39;on blast&#39;&#39;&#39;&#39; period, overheating of the burner surface, and the occurrence of great differences in temperature in the hot blast main. The improvement consists in that during the on blast period cold air is supplied through the burner, to be mixed with the hot blast inside the stove. Preferably the supply of blast air into the stove and of mixing air through the burner is controlled to obtain a constant quantity of hot blast per unit time, of a constant temperature.

United States Patent Vroege Feb. 15, 1972 [54] METHOD FOR OPERATING A REGENERATIVE GAS HEATER, AND A GAS HEATER FOR USE IN THIS [56] References Cited UNITED STATES PATENTS 2,175,611 10/1939 Linder ..263/19 3,082,995 3/1963 Krapf ..263/ l Primary Examiner-Edward G. Favors Attorney-Hall & Houghton [57] ABSTRACT In a hot blast stove for a blast furnace a method and means are provided to prevent, during the on blast period, overheating of the burner surface, and the occurrence of great differences in temperature in the hot blast main. The improvement consists in that during the on blast period cold air is supplied through the burner, to be mixed with the hot blast inside the stove. Preferably the supply of blast air into the stove and of mixing air through the burner is controlled to obtain a constant quantity of hot blast per unit time, of a constant temperature.

6 Claims, 2 Drawing Figures METHOD [72] Inventor: Aart Kornelis Vroege, Beverwijk, Netherlands [73] Assignee: Koninkliike Nederlandsche Hoogovens en StaalIabrieken N.V.

[22] Filed: Jan. 5, I970 211 Appl. No.: 653

[30] Foreign Application Priority Data Jan. 16, 1969 Netherlands ..6900740 521 u.s.c|. .L ....263/19R [51] Int. Cl ....F23l9/04 [58] Field oiSearch ..263/l9,52

METHOD FOR OPERATING A REGENERATIVE GAS HEATER, AND A GAS HEATER FOR USE IN THIS METHOD BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for operating a regenerative gas heater, for instance and in particular an air preheater for preheating the air for a blast furnace, a so called hot blast stove or cowper. Moreover this invention relates to a gas heater for use with this method.

2. Description of the Prior Art Heat regenerators or so-called gas or air preheaters are known in several fields of the art for regenerating heat generated during some process or for regenerating combustible components in a gas. One of the best known applications of such air preheaters is in the blast furnace field, where the latent heat present in coke oven or blast furnace gas or in other fuel such as natural gas is transferred into sensible heat by combustion in a burner stack or shaft, which heat is stored in a pile of porous bricks (the checker work). In a next phase of operation the air for the blast furnace is preheated by guiding it through the heated preheater in the reverse direction.

As the air in the preheater under present practice is heated to gradually increasing temperatures, the consumption of coke per ton of pig iron decreases and the production capacity of the blast furnace increases.

For this reason the aim under present practice is to obtain the highest possible preheating temperatures for the air.

In order to obtain a continuous supply of hot air to a blast furnace it is usual in practice to have three or four air preheaters or hot blast stoves per blast furnace, and therewith it is made certain that at any moment hot air from at least one of the preheaters is available for the blast furnace. Even so, however, additional measures should be taken to keep the temperature of the air after preheating at a constant value. This is so because substantial variations in the air temperature would cause a nonuniform operation of the blast furnace, which could cause disturbances or even interruptions of operation and a lower efficiency of the furnace.

As during the preheating period the checker work within the preheater will gradually decrease in temperature the air issuing from the preheater will also become gradually colder. In order to obtain air of constant temperature for supply to the blast furnace it is known in practice to mix a varying quantity of cold air with the hot air so that the ultimate mixture has a constant temperature which about corresponds to the temperature of the air leaving the air preheater at the end of the preheating period (the on blast period), at which end this air is heated to a relatively low temperature.

Several systems for this mixing of cold air with hot air are known. A quite usual system is one in which cold air for mixing is supplied to the manifold or collecting duct into which the several air preheaters dispense their heated air (the socalled hot blast main), said mixing air being supplied to that end of said main which is opposite to the end connected to the blast furnace. A disadvantage of this method consists in that the part of this main between the first and the last air preheater when switching these preheaters periodically will take at one moment the temperature of the hottest preheated air and at another moment the temperature of the cold mixing air. As the air temperatures used gradually increase (in the last few years they have been raised from about 600 to about l,l C. or even higher) this causes serious problems in design and maintenance. The steel hot blast main and the refractory lining thereof will expand and shrink considerably, and moreover this will take place in a manner which differs from place to place along the main. Also the joint of the air preheaters to this main causes serious complications by this phenomenon.

Another structure has been suggested in which the supply of the cold mixing air takes place in the hot blast main at a short This entrains that the remainder of this main will maintain a substantially constant temperature. so that the said disadvantages of expansion and shrinking will not be very serious.

A disadvantage of this structure is, however, to be seen in the fact that a complicated and expensive mixing chamber appears to be necessary in order to obtain good mixing of hot preheated air and cold air at a short distance before the entry of the air into the blast furnace. This mixing chamber is difficult to design and manufacture properly also because it i; subjected to high-thermal stresses.

Repairs to the mixing zone are, in both systems, only possible by taking all preheaters and thus also the blast furnace out of operation.

A further earlier suggested method consists in that cold air is mixed to the preheated air in more than one mixing chamber, each mixing chamber being positioned between an air preheater and the hot blast main. This gives a substantially constant temperature of the said main, and there are less serious problems in designing manufacturing and mounting the mixing chamber etc., than in the case of a common mixing chamber at a short distance before the blast furnace, but this solution also has considerable disadvantages. Particularly this solution is quite expensive because three or four separate mixing chambers are necessary.

SUMMARY OF THE INVENTION The present invention aims at removing the said objections of prior proposals. As will be explained below the application of the invention moreover gives additional advantages.

In view of the above the present invention consists in that during the on blast" (air preheating) period of the preheater cold air is introduced through the supply for the combustion air into the burner stack of the preheater and is in said stack mixed with the hot air. It is remarked that the supply for the combustion air is already present on existing air preheaters for forming a combustible mixture in the burner stack during the heating up of the air preheater (on gas" period). As mixed air thus enters the hot blast main this main will remain at a substantially constant temperature.

An important advantage of the new method moreover consists in that a good mixing within the burner stack is obtainable so that the provision of additional mixing chambers is superfluous. In this respect it is remarked that as a rule the burners applied are of a type in which the combustion air causes heavy turbulence in the burner stack. The supply of the cold mixing air through said burners thus assures a good mixing with the hot air within the burner stack.

With the usual way of operating such air preheaters the burner during the heating-up (on gas") period is cooled by the combustion air flowing through it and along it. However, during the air preheating (on blast).period the head or end face of the burner is heated to a very high temperature by the hot air which flows downwardly through the burner stack. When applying the method according to the present invention the burner head will be cooled by the mixing air also during the preheating period (on blast) so that this burner is much less subjected to thermal shocks.

A particularly great advantage is also that the valves for the preheated air, which without applying the invention should be able to withstand the highest occurring air preheating temperature in the preheater, will, when applying the invention, never obtain a higher temperature than for instance 150 C. below this highest temperature. With the gradually increasing preheating temperatures said valves cause a very difficult problem and the necessity of expensive structures in order to have them operate reliably.

When applying the invention it is possible and it has ap' peared to be particularly advantageous to vary the quantities of preheated air and cold mixing air during the onblast period I in such a way that a constant delivery of mixed air with a constant temperature leaves the air preheater.

Apart from the method described this invention also relates distance thereof before its connection to the blast furnace. to thepreheaters which are used for realizing this invention.

Such a preheater is according to this invention characterized in that the supply duct for the combustion air is connected to means for blowing air against the pressure of the air preheated in the preheater into the burner stack in a quantity which is controllable from to 20 percent of the total quantity of preheated air leaving the preheater. By this controllability it is obtainable that the mixed air has a constant temperature. The said means could consist of a separate compressor, but it is also possible to have the cold air supplied by the same compressor which also compresses the air to be preheated, which flows through the pile of bricks (checker work).

Known preheaters have either interior or exterior burner stacks. Both types could have exterior burners mainly from metal or they could have ceramic burners positioned in the burner stack. When applying the invention air preheaters with a so-called ceramic burner are preferred. Such a ceramic burner appeared to be able to give a better mixing within the burner stack over a shorter length of this stack. Thereby it is possible to arrange the outflow opening for the preheated air according to this invention at a short distance above the burner head. The height of that part of the burner stack, along which considerably varying temperatures could occur during the on gas and on blast periods will thus be very small. Thereby damage to the refractory lining is reduced to a minimum. An additional advantage is moreover to be seen in the fact that the short zone, to which possible damage to the refractory lining is restricted, is positioned immediately above the burner head, a zone where repairs within the air preheater are most easy to realize. In this respect it is noted that it has appeared possible to disconnect an air preheater temporarily from several ducts, to which it is connected and to make repairs thereon in a short period.

It is remarked that in the preceding specification the invention has...particularly been explained with reference to the use of air preheaters (hot blast stoves) for blast furnaces.

As, however, the application of a similar type of preheaters is also possible in other applications in the art in which corresponding problems have to be solved, it will be clear that the invention is not restricted to the application of preheaters for blast furnace operation.

BRIEF DESCRIPTION OF THE DRAWINGS This invention in a preferred embodiment will now be explained with reference to the annexed drawings, wherein FIG. 1 shows diagrammatically an air preheater (hot blast stove) with internal burner stack and interior ceramic burner in longitudinal section, and

FIG. 2 shows diagrammatically an apparatus arrangement for carrying out the invention.

DETAILED DESCRIPTION In FIG. 1 of the drawing reference numeral 1 shows the easing of the air preheater, which, together with the bottom 2, upon which it rests, forms a masonry structure. The brick casing is surrounded by a steel outer lining (not shown). A set of piles or columns 3 upstanding from the bottom 2 carries a grid 4 which serves for supporting perforated bricks piled thereon (checker work), which have not been shown, and through which alternately hot combustion air flows downwardly (on gas period) and air to be heated flows upwardly (on blast period). This so-called pile of bricks is separated by a separating wall 5 from the burner stack which in its turn is bordered at its other side by a segment of casing 1.

The air preheater at its upper end is closed by a brick dome 6 and a steel dome 7.

The gaseous fuel and the air for generating the hot combustion gases are supplied by a passage 8 and by annular duct 9 respectively, guiding these gases to burner block 10. The

burner has been shown diagrammatically in the drawing. It is a so-called ceramic burner known as such and arranged entirely within the lining l of the preheater in the lower end of the burner stack.

Above the burner fuel and air from the burner are intimately mixed and this fuel is thus burnt in the burner stack. The hot combustion gases are guided over the top of the separating wall 5 to leave the burner stack at the top thereof, and then they flow downwardly. After the bricks in the checker work have been heated sufficiently the supply opening 14 for the fuel passage 8 of theburner is closed. The air to be heated is thereupon supplied by means for blowing air as above described, shown at 16, FIG. 2, through opening 11 to flow ii. the opposite direction through the heater and this air is allowed to leave it through port 12 which has a valve for the preheated air, as above described, as illustrated at V. Cold air suitably supplied, e.g., by blower means I7, FIG. 2, remains flowing through opening 15 and passage 9 to burner block 10.

This cold air flowing from burner block 10 mixes intimately with the hot air flowing from above downwardly through the burner stack, after which the mixtureis guided through port 12 to a collecting duct of manifold (hot blast -main, not shown), from where it flows to the blast furnace. Port 12 with its air valve V, FIG. 2, at the outside, is arranged in the wall of casing l at the shortest possible distance above the burner head 10. The mixing zone proper is thus restricted to that part of the height of the burner stack which is between the burner head 10 and this port 12. Thereby the refractory bricks of the burner stack are subjected as little as possible to thermal shock. The cold mixing air which flows through the burner head 10 also takes care of cooling this head during the on blast period, so that this burner head both during the on gas period and the on blast period is cooled. Preferably the burner head 10 is made of ceramic material. In other respects the shape of this burner head is not essential for the present invention.

One form of apparatus suitable for carrying out the abovedescribed method aspect of the invention is diagrammatically indicated in FIG. 2, wherein the elements 1-15 correspond with those of FIG. 1, and the apparatus comprises means by which the quantities QI and Q2 per unit time of cold gas introduced through 15 and of gas to be heated introduced through 11 are varied during operation so as to obtain a substantially constant quantity (Q=C) of themixture thereof per unit of time leaving the heater via 12, at a substantially constant temperature (T=C), the variation in this embodiment being controlled so that the quantity of cold gas 01 constitutes an amount, in the range of from 0 to 20 percent of Q as aforesaid, sufficient to adjust the temperature T of the gas mixture to a predetermined value (T=C), while the quantity Q2 of gas being heated constitutes the remainder of the constant quantity (Q=C). Various forms of temperature controlled flow proportioning systems are of course well known in the art of fluid flow control, and the details of the particular systems selected form no part of the present invention, wherefore the system indicated diagrammatically in FIG. 2 is to be considered illustrative but not restrictive of systems usable to practice the invention. In this form the temperature responsive flow proportioning means 20 is controlled by temperature responsive means 21 responsive to the temperature T of the mixed gas leaving the port 12 during the on blast period (when valve V is open), and operates suitable means, shown as throttle valves 22 and 23, to vary the amounts of cold and hot gases 0] and Q2 being mixed to form the quantity Q; the quantity Q1 of cold gas from 17 of course being at its maximum, e.g., 20 percent of Q, when the checker work is hottest, and being of course reduced, with corresponding increase in the quantity Q2 of gas from 16, as the checker work cools during the progress of the on blast period.

I claim:

1. A method for operating a regenerative gas heater, in particular for preheating the air for a blast furnace (hot blast stove), characterized in that during the heating of this gas (on blast) cold gas is introduced through the supply for the combustion air to a burner in a burner stack and is mixed with the heated gas in this stack.

2. A method according to claim 1, characterized in that the quantities per unit time of cold gas introduced through said supply for combustion air and of gas to be heated as supplied to the gas heater are varied during operation so as to obtain a constant quantity of the mixture thereof per unit time leaving the heater at a constant temperature.

3. A regenerative gas heater, having a burner, of which a supply duct for combustion air is connected to means for blowing a gas such as air into a burner stack of the heater against the pressure of the gas to be heated therein, means being provided to control the quantity of said gas blown into the burner stack in a quantity region per unit time of 0 to percent of the quantity per unit time of the heated gas leaving the heater. 

1. A method for operating a regenerative gas heater, in particular for preheating the air for a blast furnace (hot blast stove), characterized in that during the heating of this gas (on blast) cold gas is introduced through the supply for the combustion air to a burner in a burner stack and is mixed with the heated gas in this stack.
 2. A method according to claim 1, characterized in that the quantities per unit time of cold gas introduced through said supply for combustion air and of gas to be heated as supplied to the gas heater are varied during operation so as to obtain a constant quantity of the mixture thereof per unit time leaving the heater at a constant temperature.
 3. A regenerative gas heater, having a burner, of which a supply duct for combustion air is connected to means for blowing a gas such as air into a burner stack of the heater against the pressure of the gas to be heated therein, means being provided to control the quantity of said gas blown into the burner stack in a quantity region per unit time of 0 to 20 percent of the quantity per unit time of the heated gas leaving the heater.
 4. A gas heater according to claim 3, characterized in that this is an air preheater (hot blast stove) having an interior burner stack and an interior so-called ceramic burner.
 5. A gas heater according to claim 4, characterized in that the opening through which the heated gas (preheater air) leaves the heater is positioned closely above the top face of the burner.
 6. A gas heater according to claim 3, characterized in that the opening through which the heated gas (preheated air) leaves the heater is positioned closely above the top face of the burner. 